Fine-tune the realtime auto-save to prevent losing unsaved content

Currently, the realtime editing auto-save is triggered once per minute, more or less. This means that the user can still lose unsaved changes if:

• they are editing alone
• they leave the editing session before the auto-save is triggered

We need to improve this to reduce the chances of losing unsaved content. Some ideas:

• trigger auto-save more often, but we need to think about the number of document versions created; we could also make the auto-save delay configurable
• trigger auto-save immediately when the editing area loses focus (if there are unsaved changes); this will cover the case when the user switches the browser tab or window
• trigger auto-save when reconnecting to the realtime editing session (if there are unsaved changes); this should cover the case when the user is disconnected, due to some network issue, after making some change, but before the auto-save is triggered; of course, this won't cover the case when the user leaves the edit mode while disconnected, but the leave confirmation popup should warn the user at least